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WE ARE NOW IN THE SHANK OF MAY Sweeps. Enjoy the programming bounty while you can. In two weeks we enter an unusually arid summer. The networks have abandoned their five-year effort to tantalize viewers during the hot season with a sprinkling of new shows. It's easy to understand why they are retreating to reruns: the last summer series to carve out an audience was Northern Exposure in 1990. If you are a habitual TV watcher, this may be a good time to take up pottery or golf or some other time-consuming hobby, because the networks' attitude this summer is: "Let them eat leftovers."